1. Field of the Invention
Exemplary embodiments of the present invention relate to a tandem-type image forming apparatus for transferring toner images formed on multiple image carriers onto a belt member or a recording medium to form a composite color image.
2. Discussion of the Related Art
Tandem-type image forming apparatuses generally have a transfer nip that is a contact area formed between each of multiple image carriers and a belt member held in contact with the multiple image carriers. Respective single-color toner images formed on the multiple image carriers are transferred sequentially onto either the belt member or a recording medium carried on the belt member at the respective transfer nips to form a multi-color or composite color toner image.
In this configuration, the belt member is driven at a constant speed by a belt drive motor. However, the speed of the belt member can vary due to, for example, eccentricity of extension rollers that extend to tension the belt member, eccentricity of drive gears, and/or uneven thickness in a circumferential direction of the belt member. If the speed of the belt member varies during a primary transfer operation for overlaying toner images on the belt member, the colors of the toner images may be displaced or shifted, resulting in significant deterioration in image quality or production of defective images.
Some image forming apparatuses employing a tandem-type image forming units include technologies to suppress occurrence of the above-described color shift caused by fluctuation in the velocity of the belt member. For example, the image forming apparatus may include a belt member with a scale formed thereon. The scale includes multiple marks formed at given pitches in the circumferential direction of the belt member. The image forming apparatus also includes a scale sensor to detect the multiple marks of the scale. A controller then detects the speed of the belt member based on time intervals between successive detections of the scale marks, and based on the detection results, controls the drive speed of a belt drive motor to reduce fluctuation in the velocity of the belt member.
Although generally successful, the related-art image forming apparatus having the above configuration cannot completely reduce or prevent color shift because fluctuation in the velocity of the belt member does not occur equally over the entire circumferential length of the belt member, as illustrated, for example, in FIG. 1.
FIG. 1 illustrates a configuration of a transfer unit 200 incorporated in a related-art image forming apparatus.
The transfer unit 200 includes a transfer belt member 201 that is supported by a drive roller 202 disposed to an inner surface of the transfer belt member 201, a driven roller 203, a tension roller 204, and four primary transfer rollers 205a, 205b, 205c, and 205d. With this configuration, the surface speed of the belt member at each of the transfer nips formed between photoconductors 210a, 210b, 210c, and 210d and the primary transfer rollers 205a, 205b, 205c, and 205d, respectively, depends on which area of the transfer belt member 201 enters the position where the transfer belt member 201 is wound around the drive roller 202.
Specifically, the transfer belt member 201 may have at least an uneven thickness in the circumferential direction thereof. Further, the drive roller 202 and the tension roller 204 may be slightly eccentrically mounted on their shafts. In addition, the speed of the transfer belt member 201 at each transfer nip can also fluctuate due to the eccentricity of the drive roller 202. Therefore, the fluctuation in the velocity of the transfer belt member 201 that is observed at each transfer nip is a combination or superimposition of a component of fluctuation in the belt velocity due to the thickness fluctuation of the transfer belt member 201 and a component of fluctuation in the belt velocity due to eccentricity of the drive roller 202.
Accordingly, the surface speed of the transfer belt member 201 may vary at each transfer nip. However, the surface speed of the transfer belt member 201 may be different at the transfer nips and in a belt tensioned area between the tension roller 204 and the driven roller 203, because the speed of the transfer belt member 201 varies in the belt tensioned area due to the fluctuation in belt thickness at the belt wound area of the drive roller 202 and the eccentricity of the drive roller 202 and due to the fluctuation in the belt thickness at the belt wound area of the tension roller 204 and the eccentricity of the tension roller 204. In the belt tensioned area, the velocity fluctuation in which these velocity fluctuations are superimposed onto each other may occur on the surface of the transfer belt member 201.
Further, it is typical to provide a contact and separation mechanism to move the transfer belt member 201 into and out of contact with the photoconductors for yellow, magenta, and cyan toner images. In the configuration with the contact and separation mechanism, regardless of the operations performed by the contact and separation mechanism, it is preferable that the scale sensor detects the multiple marks of the scale formed on the transfer belt member.